For broadband wireless networks that support mixed traffic, it is required that a medium access control (MAC) be able to effectively adapt to changing channel conditions and traffic requirements. One of the key resources in wireless communication is the spatial diversity provided by antenna arrays. Efficient exploitation of spatial diversity is fundamentally important to resource critical wireless applications. See Tsoulos, “Smart Antennas For Mobile Communication Systems: Benefits and Challenges,” Electronics and Communication Engineering Journal, Vol. 11(2), April 1999.
One of the most aggressive ways of exploiting the spatial diversity is space-division multiple-access (SDMA), or spatial multiplexing, that attempts to multiply the throughput of a wireless network by introducing “spatial channels”. The benefits of SDMA in narrowband applications have be investigated by Shad et al., “Indoor SDMA Capacity Using a Smart Antenna Base Station,” IEEE Proc. ICUPC'97, pp. 868—872, 1997; Farsakh et al., “On the Mobile Radio Capacity Increase Through SDMA,” Accessing, Transmission, Networking Proceedings, pp. 293-297, 1998; Xu et al., “Throughput Multiplication of Wireless LANs for Multimedia Services: SDMA Protocol Design,” Proc. Globecom'94, San Francisco, Calif., November 1994; Ward et al., “High Throughput Slotted ALOHA Packet Radio Networks with Adaptive Arrays,” IEEE Trans. COM, Vol. 41(3), pp. 460-470, March 1993. The narrowband SDMA scheme is particularly suitable for fixed wireless networks where the spatial characteristics are relatively stable. For a system with M antenna elements, the underlying idea is to allow M subscribers to share a time-division multiple-access (TDMA) slot through M spatial channels/slots, so that the total number of non-interfering traffic channels can be increased by M fold.
Though intuitively promising, an obvious flaw of this scheme is that spatial channels are rarely orthogonal in practice. In other words, traffic over SDMA channels are mutually interfering. If multiple subscribers are assigned to one time slot without considering these spatial characteristics, the one with an unfavorable spatial configuration may experience significant throughput disadvantages. Since the effectiveness of spatial separation depends on the base-station array responses (often referred to as the spatial signatures) of all co-slot subscribers and the spatial processing technique employed, the instantaneous signal-to-noise-plus-interference ratio (SINR) of spatially multiplexed outputs can vary dramatically. This problem is investigated by Ward et al., “High Throughput Slotted ALOHA Packet Radio Networks with Adaptive Arrays,” IEEE Trans. COM, Vol. 41(3), pp 460-470, March 1993, where collision due to un-resolvable packets (when the arrival angles of multiple packets are within a threshold) is accounted for. Although the analysis is somewhat simplified, the study reveals some of the key limitations of basic SDMA scheme.
A fundamental solution to the above problem is the “channel-aware” MAC protocol that assigns traffic channels based on the spatial characteristics of the subscribers. Using such a protocol, the performance of SDMA system can be enhanced with spatial signature-based scheduling (e.g., assigning the “less-interfering” subscribers to the same time slot to increase the traffic throughput). The MAC treatment allows a system to exploit the spatial diversity in an efficient manner using spatial processing with fixed-complexity. Several scheduling algorithms are proposed and studied for “narrow band” systems where the spatial characteristics can be described by a one-dimensional vector; see Shad et al., “Indoor SDMA Capacity Using a Smart Antenna Base Station,” IEEE Proc. ICUPC'97, pp. 868-872, 1997; Farsakh et al., “On the Mobile Radio Capacity Increase through SDMA,” Accessing, Transmission, Networking Proceedings, pp. 293-297, 1998; and U.S. Pat. No. 6,041,237, “Method of Channel Allocation,” issued Mar. 21, 2000.
All of the existing channel allocation schemes, however, consider “narrowband” TDMA wireless network in conjunction with SDMA. None of them is applicable to broadband wireless networks. The prime reason is that in broadband application, subscribers' spatial channels are two-dimensional, in both space and frequency, and channel assignment under spatial multiplexing becomes a more involved problem.
Recently, there is an increasing interest in frequency division multiple access (OFDMA) based broadband wireless networks. OFDMA can be viewed from one perspective as attempting to achieve system capacity over multiple-subscriber broadband wireless channels.